As an encapsulating material of an LED device, an epoxy resin has generally been used. However, modulus of elasticity of the epoxy resin is high, so that under stress by temperature cycles, a bonding wire may be disconnected or cracks may generate at the epoxy resin in some cases. Also, because of the stress that the epoxy resin gives to the LED device, a crystal structure of a semiconductor material collapses and lowering in luminous efficiency is concern.
As a measure thereof, a method of using a silicone-modified organic resin, a method of adding silicone fine particles to the epoxy resin, etc., have been proposed (see Patent Literature 1 and Patent Literature 2). However, these methods use encapsulants containing an organic component with a larger amount, the cured composition causes cracking by the UV light having a short wavelength emitted from the photosemiconductor, and yellowing proceeds when it is used for a long period of term which leads lowering in brightness of the LED apparatus with a lapse of time.
Further, as a flexible encapsulant containing no organic component, it has been proposed to use a silicone resin (for example, see Patent Literature 3 to 5). The silicone resin is excellent in heat resistance, weather resistance and color fastness as compared with those of the epoxy resin. In particular, it is excellent in transparency, optical characteristics, etc., as compared with the other organic materials such as the epoxy resin, etc., so that, in recent years, it has been increasingly used mainly in a blue LED and a white LED. Further, in the silicone resin composition having a flexible molecular structure, it has a rubber elasticity with a wide range from low temperature to high temperature, so that it can effectively suppress crack against thermal shock, whereby an LED apparatus having high reliability with a long term can be obtained.
On the other hand, such a silicone resin material generally has a tack (tackiness), and is a rubber material so that the material strength is weak. For the purpose of supplementing the above, it has been generally known that improvement in strength of the material and improvement of tackiness can be done by adding a filler.
A resin composition for encapsulating a photosemiconductor having high hardness and heightened impact resistance has been proposed by formulating a fine particulate high molecular weight polyorganosilsesquioxane having an average particle diameter of 100 nm or less to a silicone resin composition (see Patent Literature 6).
Also, a resin composition for encapsulating a photosemiconductor having high hardness and heightened impact resistance has been proposed similarly by adding silicone series polymer particles having a silicone core-alkoxysilane condensate shell structure in which silicone particles having an average particle diameter of 0.001 to 1.0 μm is coated by an alkoxysilane condensate to a silicone resin composition (see Patent Literature 7 and 8, etc.).
Further, there is a prior art technique in which silicone fine particles are filled in a silicone resin composition for the purpose of improvement in strength of a material and improvement in light extraction efficiency (see Patent Literature 9 to 11).